Question

Name Lab Time 357 Pre-Lab Quiz Buffers & Group I Cations Chemistry 1412 1. Which of the following are characteristics of a buffer? (YOU MAY CIRCLE MORE THAN ONE POSSIBLE ANSWER) а solution composed of a weak acid and the salt of the acid solution composed of a strong acid and strong base solution which resists changes in pH only if an acid is added solution which resists changes in pH only if a base is added solution which resists changes in pH if an acid or base are added b. 2. The pH of a 0.0795 M monoprotic acid (HA) solution is 2.705 a) What is the [H'] in the solution? b) What is the [A]? HA CHA) c) What is the value of Ka for HA? 3. Which of the following are included in the Group I cation classification? Pb b. Cl

Answer 1

1)

A buffer is a solution containing a weak acid + its strong conjugate base (salt) or weak base and its strong conjugate acid (salt).

Further, a buffer resists the change in pH in both cases- Addition of acid or addition of Base.

Thus, the answer will be (a) and (e)

2)

The equilibrium is,

$HA\rightleftharpoons H^++A^-$

a)

For a monoprotic acid, pH is given as,

$pH=-\log_{10}([H^+])\\ \Rightarrow [H^+]=10^{-pH}\\ \Rightarrow [H^+]=10^{-2.705}M=1.97\times 10^{-3} M$

b)

For a monoprotic acid, the concentration of [A^-] and [H⁺] are equal. Thus,

$[A^-]=[H^+]=1.97\times 10^{-3} M$

c)

Here, the initial concentration of Acid is 0.0795 M

But some of it has dissociated into H⁺ and A^-. Thus, the concentration remaining will be,

[HA] = Initial - [H⁺] =0.0795 M - 1.97 x 10^-3 M

K_a of a monoprotic acid is given as,

$K_a=\frac{[H^+][A^-]}{[HA]}=\frac{(1.97\times10^{-3})(1.97\times10^{-3})}{0.0795-(1.97\times10^{-3})}=5.02\times 10^{-5}$